Packaging containers of the single use disposable type for liquid foods are often produced from a packaging material of the type mentioned above. One such commonly occurring packaging container is marketed under the trademark Tetra Brik Aseptic® and is principally employed for liquid foods such as milk, fruit juice etc. The packaging material in this known packaging container typically comprises a core layer of paper or paperboard and outer, liquid-tight layers of thermoplastics. In order to render the packaging container light- and gas-tight, in particular oxygen gas-tight, for example for the purpose of aseptic packaging and packaging of fruit juices, the material in these packaging containers is normally provided with at least one additional layer, most commonly an aluminum foil, which moreover renders the packaging material thermosealable by inductive thermosealing which is a rapid and efficient sealing technique for obtaining mechanically strong, liquid- and gas-tight sealing joints or seams during the production of the containers.
Packaging containers are generally produced by means of modern packaging machines of the type which form, fill and seal packages from a web or from prefabricated blanks of packaging material. From, for example, a web, packaging containers are produced in that the web is reformed into a tube by both of the longitudinal edges of the web being united to one another in an overlap joint. The tube is filled with the intended liquid food product and is divided into individual packages by repeated transversal seals of the tube at a distance from one another below the level of the contents in the tube. The packages are separated from the tube by incisions in the transverse seals and are given the desired geometric configuration, normally parallelepipedic, by fold formation along prepared crease lines in the packaging material.
In the prior art, packaging laminates for this kind of packages usually have an innermost, inside thermoplastic heat sealing layer, most commonly of a low density polyethylene (LDPE), which normally has adequate properties for heat sealing and for the function of moisture barrier towards the filled liquid contents of the package.
By innermost or inside layer is meant a layer which is applied on the side of the packaging laminate facing towards the inside of a packaging container formed from the laminate, and which will be in contact with the filled contents of a filled packaging container.
From the consumer's point of view, it is desirable that the packaging container be easy to handle and easy to open when it is time to empty the package of its contents, and in order to satisfy this need, the packaging container is often provided with some type of opening arrangement, with the aid of which it may readily be opened without the need to employ scissors or other implements.
A commonly occurring opening arrangement in such packaging containers includes a hole punched in the core layer of the package wall, the hole being covered on the inside and outside of the package wall, by the respective outer layers of the packaging wall which are sealed to one another in the region of the opening contour of the through-going hole, thus forming a membrane of the layers not being paperboard. One example of a prior art opening arrangement has a separate pull-tab or opening strip which is applied over the hole and which is rupturably sealed to the outer layer of the outside of the package wall along a sealing joint around the entire opening contour of the hole and at the same time permanently sealed to the outer layer in the region inside the opening contour of the hole.
In more advanced opening arrangements, an opening device, usually of moulded plastics, having a pouring spout and a screw top for resealing, is applied onto the region of and around the hole, which opening device is designed to penetrate or remove the membrane within the hole region by a pushing-down or screwing-down movement or, alternatively, to remove the membrane by a screwing- and/ or pulling-up movement of the opening device. In the latter kind of opening device, the inside of a screwable part of the opening device is adhered to the membrane of the hole, in such a way that when it is screwed upwards away from the packaging wall, the membrane is lifted along with the screwable part and torn away from the edges of the hole, leaving a practically clean-cut hole for pouring the filled contents out of the package.
In particular, the latter kind of opening arrangement may function similarly to a screw top of a bottle and is often desirable, since it avoids pushing residues of the membrane down through the hole into the package and the filled product.
A precondition for such an opening arrangement to function efficiently and expediently is that, there is adequate adhesion between the different layers of the membrane such that it does not delaminate when screwing- and/ or pulling-up forces, or forces of screwing and/or pushing-down, are applied to it during the opening operation.
It is generally difficult to obtain such adequate adhesion within the regions of the holes, because of the difference in total laminate thickness between the regions of the holes and the regions outside of the holes when laminating together the aluminum foil and thermoplastic layers of the membrane. When passing a web of the laminated layers through a press nip in a lamination station, the layers are pressed to adhere to each other by means of a pressure roller and a cooled cylinder. In the regions defined by the hole or the slit, the press nip is unable to press the aluminum foil and the polymer layers together sufficiently for achieving the requisite adherence.
Thus, the thickness variations of the core layer may cause that the aluminum foil, which is relatively thin, will not be pressed against and adhered sufficiently well to the surrounding layers of thermoplastics within the whole of the region defined by the hole, which means that air may be entrapped adjacent to the edges of the holes. This in turn means that there may be fracture formations in the aluminum foil, which may lead to the gas-tightness of the packaging container being impaired and thereby also the color, taste and nutritional values of the packaged food product. Furthermore, the integrity of the package may be impaired, which in turn may disturb the aseptic performance of the package.
The air inclusions also result in it being difficult to tear off or penetrate the membrane consisting of the aluminum foil and the polymeric films in the hole or slit, with the ability to open the packaging being restricted and/or with it not being possible to make a clean cut when penetrating, resulting in the formation of frayed edges.
These problems have hitherto been eliminated or at least been reduced to an acceptable level, by means of a press roller comprising a metal core with a circular-cylindrical jacket surface, which jacket surface is faced with an inner facing layer consisting of an elastic material, having a first hardness and a first thickness, and arranged on the outside of the inner facing layer, an outer facing layer consisting of an elastic material, having a second hardness and a second thickness, with the first hardness being greater than the second hardness and with the first thickness being greater than the second thickness. Preferably, the first hardness is at least 15% greater, more preferably 20% and most preferably 25% greater, than the second hardness, calculated in Shore A, with the outer facing layer exhibiting a hardness of 50-80 Shore A, preferably 60-75 Shore A.
Preferably, the second thickness constitutes 5-25%, more preferably 7-20% and most preferably 8-15%, of the total of the first thickness and the second thickness. Preferably, the second thickness is 1-10 mm, more preferably 1-5 mm and most preferably 1-3 mm.
Such a press roller has been described in a separate pending application WO 01/02751, also belonging to the applicant of the present application.
Due to the outer facing layer of lower hardness, a desired penetration is achieved in the hole regions in the core layer, when the core layer, the aluminum foil and the polymeric layers pass through the press nip, at the same time as the low thickness of the outer, softer facing layer results in the press nip length not being appreciably extended, meaning that a desired pressure can be maintained in the press nip while still retaining a high line load. The inner and outer facing layers may be made of the same or different elastomeric materials, such as for example rubber or polyurethane materials.
The described press roller should be used in at least one of the lamination station for the inside layers and/or the lamination station for the aluminum foil and the intermediate bonding layer. It may also be used in the lamination of the thermoplastic layer on the outside of the core paperboard layer.
For the purpose of an opening arrangement as described above, in particular the one functioning by a screw-pulling motion, an innermost heat sealing layer of LDPE usually is bonded to the aluminum foil by means of a bonding layer of an adhesive polymer, such as for example a graft modified polyolefin or a copolymer of ethylene and (meth)acrylic acid or a ionomer.
During the latest years, there has been an increasing interest for the use of innermost layers in packaging laminates comprising the kind of ethylene-alfa-olefin copolymers that are polymerized in the presence of a metallocene catalyst, i.e. metallocene polyethylenes (hereinafter denoted as m-PE), which normally are a type of linear low density polyethylenes (m-LLDPE).
Metallocene-polymerized polyethylenes generally have desirable properties like improved tear and puncture resistance, toughness, impact strength, clarity, antiblocking properties and heat sealing performance compared to that of ordinary LDPE. In the manufacturing of packaging containers it would therefore be highly desirable to be able to use m-PE in the innermost sealing layer in order to improve package integrity and sealability properties.
With package integrity is generally meant the package durability, i.e. the resistance to leakage of the packaging container. This is tested in a first step by measuring the electrical conductivity through the packaging laminate of the package in order to indicate if there is any kind of hole or crack in the inside thermoplastic layer(s). In a second step, the size and shape of the hole or crack is studied further by immersing the packaging laminate into a red ink solution, whereby the paperboard core layer will be colored in red at and around the hole or crack. The result is reported in number of containers with leakages out of 3000 tested packages. However, the test result does not automatically mean that the containers would actually be leaking out the filled contents, because the test method is more severe and shows very fine cracks and holes that would perhaps not normally cause problems in real, daily storage and use. In general, it has been seen that by using m-PE in the innermost sealing layer instead of LDPE, the package integrity is at least equally reliable even when employing thinner layers of the m-PE than the LDPE.
With sealability properties are meant the ability to heat seal appropriately within a temperature interval or interval of power supply. There, are for example three heat seals in a normal parallellepipedic packaging container of the Tetra Brik Aseptic® type, i.e. the seal transversally of the tube, the longitudinal overlapping seal along the tube and the seal of the longitudinal strip over and along the longitudinal seal on the inside of the tube. The transversal seal involves the double thickness of the packaging laminate and demands the highest power for sealing. It has generally been seen that the “window” or interval of temperatures and power supply within which adequate sealing is performed is greater for an m-PE than for normal LDPE heat sealing inside layers. For example, in a Tetra Brik Aseptic® filling machine, (such as TBA/8), which seals the packages by inductive sealing, the power supply to the inductive heating unit, is measured by scale units. The result of the test is reported as the range of scale units, within which adequate sealing is performed. Thus, for an m-PE inside material that is comparable to an equivalent LDPE inside material, the range of reported scale units for the transversal seal is much wider. This means that it will be less critical to exactly adjust the temperature settings of the sealing part of the filing machines, which are mostly operated by personnel at diaries and packaging plants, and that the sealing operation will be more reliable and less sensitive to fluctuations in the temperature of the sealing tools.
Thus, by exchanging the innermost layer of thermoplastics in the above-described packaging laminate from the traditional LDPE to a layer comprising an m-PE in the majority, heat sealability properties as well as package integrity may be improved or, alternatively, maintained at lower amounts of heat sealable polymer.
However, when employing m-PE in the innermost layer of a conventional packaging laminate, instead of the normal LDPE, a considerable deterioration In openability of the opening device appears. Suddenly the membrane breaks between the aluminum foil and the innermost layer of thermoplastics, i.e. between the aluminum foil and the layer comprising an adhesive polymer, due to the screwing and/or pulling movement of the membrane when opening the opening device, such that part of the membrane remains, covering the hole, and impedes the pouring-out of the contents of the package. This problem appears to be caused by insufficient adhesion between the aluminum foil and the adjacent layer of adhesive polymer on the inside of the aluminum foil.
The problem has been solved in a co-pending patent application, also belonging to the applicant of the present application, by a packaging laminate comprising a core layer of paper or carton with through-going holes, openings or slits, a layer of thermoplastics applied onto one outer side of the core layer, an aluminum foil applied onto the other, inner, side of the core layer, extending throughout the laminate, and bonded to the core layer by means of an intermediate layer of thermoplastics, the two layers of thermoplastics both extending throughout the laminate and being sealed to each other within the regions of the holes to form a membrane of aluminum foil and thermoplastics, and a layer of one or more thermoplastic materials applied onto the other, inner, side of the aluminum foil, wherein the layer of one or more thermoplastic materials consists of three part-layers, being a first adhesive part-layer applied onto the aluminum foil, a second intermediate part-layer of low density polyethylene (LDPE), and a third innermost part-layer comprising in the majority an ethylene—α-olefin copolymer, polymerized in the presence of a metallocene catalyst, a so called metallocene polyethylene (m-PE).
By such a packaging laminate, having an inside structure comprising three part-layers respectively comprising adhesive polymer, LDPE and m-PE in this order, an improvement in sealability and package integrity qualities is obtained as well as required openability properties, for the purpose of a packaging container having an opening arrangement, which at opening removes the membrane of laminated layers of aluminum foil and thermoplastics from the region of a hole in the core layer, made previous to lamination.
Such a packaging laminate is especially suitable for the aseptic packaging of milk and other foodstuffs.
An improvement in sealing and package integrity properties was expected from the usage of an m-PE instead of an LDPE in the innermost layer of a package in general, since these effects of m-PE qualities are known to a person skilled in the art. By improvement is to be understood also when the amount of heat sealable polymer may be reduced at maintained sealing and package integrity properties.
However, by using m-PE in the inside layer, the above defined openability was considerably deteriorated compared to when using LDPE, and it is believed that the strength of the m-PE became too high at opening, such that the weakest point at opening would be at the interface between the aluminum foil and the layer of adhesive polymer, why delamination between these layers would occur instead of the entire removal of the hole membrane.
According to a comparative example, wherein the thermoplastic structure inside of the aluminum foil consists of a first layer of an adhesive polymer and a second innermost layer of either LDPE or m-PE, the m-PE variant left clearly more residues of the membrane within the region of the hole, while the LDPE variant resulted in a good clean-cut opened hole.
Gradually, the inventors thus developed a theory that it is important to assure that the weakest point of the membrane at opening will not be at an interface between the layers of the membrane. The force needed to break loose the membrane from the packaging laminate around the edges of the hole, should not be higher than the adhesion force between the layers, especially between the adhesive layer and the aluminum foil, since this has often shown to be the weakest point during the development work in connection with the invention. In order to move the weakest point away from this interface between layers, mainly two considerations are believed to be of importance.
Firstly, the adhesion force between the aluminum foil and the layer of adhesive polymer should be above a certain level, why it, for example, also has been seen that it is necessary to employ a layer of an adhesive polymer for the contact with the aluminum foil, which measure however, not is sufficient in itself. The choice of adhesive polymer may play a role of course, in addition to other ways of increasing the adhesion.
Secondly, the force needed to tear the membrane away from the packaging laminate around the edges of the hole should be balanced against the above adhesion force, i.e. the strength of the sealing layer (LDPE/m-PE), along the plane of the layer in the packaging laminate around the edges of the hole, should be below a certain level in relation to the adhesion strength between aluminum foil and adhesive layer.
Furthermore, these two considerations for openability should be balanced to match the requirements on improved sealing and package integrity properties from the usage of m-PE in the sealing layer(s).
While investigating this theory, it was thus unexpectedly found that a co-extrusion coated combination of an innermost layer comprising m-PE with an intermediate layer of LDPE and a layer of an adhesive polymer applied towards the aluminum foil, enabled adequate openability properties as described above, at the same time as improved sealability and improved or maintained package integrity properties were obtained.
In a test series a packaging laminate having a two-layer inside structure was comparable to a packaging laminate having a three-layer inside structure.
The packaging laminates had the structure of an outer layer of about 16 g/m2 of LDPE, a paperboard core layer, an intermediate bonding layer of about 23 g/m2 of an LDPE, an aluminum foil, and the inside structure. The two-layer inside structure had a layer of about 6 g/m2 of an adhesive ethylene-acrylic acid copolymer applied onto the aluminum foil and an innermost layer comprising m-PE in a quantity of about 20 g/m2 applied next to the adhesive layer. The three-layer inside structure had a layer of about 6 g/m2 of an adhesive ethylene-acrylic acid copolymer applied onto the aluminum foil, an intermediate layer of about 10 g/m2 of LDPE and an innermost layer comprising m-PE in a quantity of about 10 g/m2 applied next to the intermediate layer. The results showed a remarkable improvement for the three-layer variant in openability of a screw-top opening device of the kind that removes the membrane by a combined screwing-up and pulling-up motion. From the two-layer inside packaging laminate, almost all the packages opened at room temperature showed delaminations between the aluminum foil and the adhesive layer. The result was about the same at refrigerator temperature. However, from the three-layer inside packaging laminate, significantly fewer packages showed delaminations between the aluminum foil and the adhesive layer (8 out of 50 at both ambient and refrigerator temperature). The results from this early test series have since been improved further by optimizing the laminate structure and the lamination process.
However, the problem that the present invention intends to solve is that, for the long time packaging and storage of more aggressive or acidic foodstuffs, like for example fruit juices, the adhesion between the layers will be reduced during time, due to the migration of free fatty acids through the inside layers of thermoplastic material from the filled product into the interface between the aluminum foil and the adjacent layer of adhesive polymer. Even though the adhesion will be high initially when the packaging container is first made and filled, the adhesion between the aluminum foil and the adjacent layer of adhesive polymer as well as the openability of the packaging container will be considerably reduced after a longer time period such as a few months or a year. By the expression long-term storage is meant such a long time that an aseptic packaging container filled with fruit juice, normally should be able to be stored at ambient temperature, before the fruit juice has lost its nutritional value and taste. Normally this time is at least 6 months, preferably at least one year.